In an electronic circuit board embedded in an electronic device, soldering is used to connect electronic parts to the board. Among welding methods for joining metals, soldering is a kind of brazing and soldering which welds metals by using a brazing metal whose melting point is lower than a base (parent) metal, utilizing a wetting phenomenon in which no melting of the base metal occurs. An alloy, out of metals to be used as the brazing metal, having a melting point of less than 450° C., in particular, is called “solder”. In general, the melting temperature of solder used for outer connection of electronic devices such as a resistor, capacitor and diode or of a semiconductor package, and for secondary mounting of a flip chip package or the like is less than 250° C. at the highest and the soldering in these steps is performed by a flow or reflow method, hand soldering method, or the like. Here, in the case of pure metal, eutectic alloy, and compound, the melting temperature represents the melting point thereof and, in the case of an alloy having two or more phases, the melting temperature represents the liquidus-line temperature thereof. On the other hand, the solder used mainly for primary mounting in the inner portion of a semiconductor package is called “high-temperature solder” which does not melt at the flow or reflow temperature (about 260° C.). Soldered portions in the above cases are loaded with thermal fatigue caused by a temperature rise or drop occurring at the time of on/off operations of devices and, therefore, thermal fatigue property is used conventionally and generally as an index of connection reliability in the soldered portion. Moreover, in the application of the soldering process to the mobile device recently typified by a mobile phone, such a property is required that the soldered portion of the mobile device can withstand the drop impact that occurs when the mobile device is dropped. As a result, a new index of connection reliability such as drop impact resistance in the soldered portion has become necessary.
Conventionally, as a material for a solder alloy, components comprising tin (Sn) and lead (Pb) have been widely used. However, in order to meet the environment requirements in recent year to comply with the RoHS (Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment) instructions in the EU (European Union), a lead-free solder comprising no Pb has been widely developed and practically used. As a lead-free solder material, an Sn—Ag—Cu alloy is now one of promising candidate materials (see Japanese Patent Application Laid-open No. Hei 5-50286 (“JP '286”)). In the electronic parts mounting processes, the lead-free materials such as Sn-3.0 Ag-0.5 Cu and Sn-4.0 Ag-0.5 Cu alloys are being used normally.
In recent years, as high-density mounting of electronic parts progresses, areas of electrode pads on the board are being reduced rapidly and, therefore, it is necessary to decrease the amount of solder to be used at a weld, thus causing a further increase in a load imposed on the soldered portion where not only lead solder but also the lead-free solder is used. In the soldered portion, in particular, where outer connection of a semiconductor package is made or parts such as a resistor, capacitor and diode are connected on a mother board, a requirement is becoming large that additional connection reliability represented as drop impact resistance has to be satisfied. In order to solve these problems, by adding various elements, an attempt to improve connection reliability in the soldered portion has been studied.
A lead-free solder alloy is disclosed in Japanese Patent Application Laid-open No. 2002-239780 (“JP '780”) which has a silver (Ag) content being lower than that of the normal lead-free solder described above and provides an excellent drop impact resistance when a specified amount of Ag is contained. That is, the lead-free solder alloy contains 1.0 to 2.0% by mass of Ag, 0.3 to 1.5% by mass of copper (Cu), a remainder of Sn and unavoidable impurities. This enables the excellent thermal fatigue resistance and impact resistance to be provided to the lead-free solder. Further, it is disclosed in the Patent Document 2 that, by adding, by mass %, 0.05 to 1.5 nickel (Ni) or 0.005 to 0.5 iron (Fe), strength of a solder alloy can be enhanced.
Also, in Japanese Patent Application Laid-open No. 2005-319470 (“JP '470”), a lead-free solder is disclosed which is obtained by allowing an Sn—Ag—Cu lead-free solder to contain fine particles having an element that does not substantially melt in the solder. This enables a structure of the solder to be made finer and its mechanical strength and its thermal impact resistance to be improved. Examples of the element that is contained in the fine particle and does not melt in the solder include boron (B), aluminum (Al), Ti, vanadium (V), chromium (Cr), manganese (Mn), Fe, cobalt (Co), Ni, zirconium (Zr), niobium (Nb), and molybdenum (Mo).
It is also disclosed in Japanese Patent Application Laid-open No. 2006-159266 (“JP '266”) that solder joint stability is improved by adding silicon (Si) and B to a high-temperature Sn—Sb—Ag—Cu solder having a melting temperature of 250° C. or more. It is also reported that the addition of the above elements suppresses the enlargement of an antimony (Sb) crystal and affinity occurs among metals at the time of melting, which prevents aggregation of metal, thus suppressing the occurrence of void.
As described above, as the high-density mounting of electronic parts progresses and mobile electronic devices become highly functional, demands for reliability in the soldered portion and for excellent drop impact resistance increase and, therefore, there is a problem that sufficient performance may not be expected from the conventional solder alloy. Particularly, as for solder alloys used for outer connection of semiconductor packages or for connection between a resistor, capacitor, diode, or the like and a mother board, the performance thereof has not been satisfactory.
Generally, in the soldering for the primary mounting (high temperature soldering) used for connection of a semiconductor package using flip chip bonding, a portion surrounding the soldered place is filled with a sealing resin. On the other hand, the soldered portion on the secondary connection (secondary mounting) side and the soldering portion where a resistor, capacitor, diode, or the like are joined to the mother board is not always filled with the resin. Therefore, in these portions not filled with the resin, it is required that the solder itself exhibits high connection reliability.
In the solder disclosed in the JP '286, its thermal fatigue property has been improved in response to the conventional demand for better performance in the soldered portion and, as a result, the drop impact resistance required for the application to mobile devices is not taken into consideration.
In the solder disclosed in the JP '780, the Ag content is reduced so as to be kept within a specified range, whereby the solder is allowed to have ductility to thereby improve its drop impact resistance. However, in the case of the disclosed solder, interfacial strength between the solder and board electrode is not sufficient, thus leading to fine pitch and, therefore, when an area and volume of the soldered portion are reduced, high drop impact resistance is not always exhibited to satisfy the requirements for solder joint on the secondary side.
In the solder disclosed in the JP '470, it is reported that the structure of the soldered portion is made finer due to the existence of fine particles of refractory elements, which can suppress crack progress, thus improving the drop impact resistance. However, there is no description about the drop impact resistance required in the case of application to the mobile devices. At the time of the occurrence of drop impact, the interfacial strength between the solder and board electrode becomes a main important factor. Nevertheless, the JP '470 only discloses such that a thickness of intermetallic compound formed between the solder and board electrode is reduced to thereby improve connection reliability therebetween. It is not considered, however, that the thickness of the intermetallic compound is the only factor to achieve excellent drop impact resistance.
In the JP '266, the solder alloy is disclosed that has improved solder joint stability by adding trace quantities of Si and B. However, generally, it is necessary that the solder used for the primary mounting of a semiconductor package using flip chip bonding has its melting temperature of 250° C. or more and, therefore, the solder disclosed in the JP '266 is not allowed to be used for the secondary mounting or for the connection with the mother board or the like. Moreover, in the primary mounting in the semiconductor package, the surrounding portion of the soldered place is filled with the sealing resin, necessity of considering the drop impact resistance is low and, therefore, in the JP '266, there is neither description nor suggestion about the drop impact resistance required for the application to mobile devices.
The present invention has been made in light of the problems described above and has an object to provide a solder alloy, a solder ball and an electronic member having solder bump that are to be used for connection with a mother board or the like, having a high drop impact resistance required in the application to mobile devices or the like and has a melting temperature of less than 250° C.